1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the same, and is applicable to an image pickup apparatus which uses a solid-state image sensor, such as a video camera, an electronic still camera, a TV camera, and a monitoring camera, or is applicable to a silver-halide film camera.
2. Description of the Related Art
As a photographic optical system used for an image pickup apparatus, there is required a zoom lens that has a short and compact total lens length, a high zoom ratio, and a high resolution.
A positive-lead type zoom lens is known. In the positive-lead type zoom lens, a lens unit having positive refractive power is arranged on an object side. As the positive-lead type zoom lens, a four-unit zoom lens is known. The four-unit zoom lens includes, in order from an object side to an image side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power.
In order to reduce a size of the entire zoom lens and achieve high resolution, a zoom lens is required to perform excellent chromatic aberration correction. That is, a zoom lens is required to effectively correct chromatic aberrations, such as axial chromatic aberration and chromatic aberration of magnification. In order to reduce a variation in axial chromatic aberration and chromatic aberration of magnification during zooming, each lens unit of the zoom lens is required to independently perform chromatic aberration correction. Therefore, in many zoom lenses, each lens unit includes one or more lenses among positive lens and negative lenses made of different materials.
The four-unit zoom lens described above includes first, second and third lens units, each of which is provided with two lenses, that is, a single positive lens and a single negative lens, which are minimally necessary for chromatic aberration correction, and a fourth lens unit, which has a small variable magnification sharing and is provided with a single positive lens.
In U.S. Pat. No. 5,859,729 and U.S. Pat. No. 7,764,438, all lens units move during zooming. U.S. Pat. No. 5,859,729 discusses a zoom lens having a zoom ratio of about 2, and U.S. Pat. No. 7,764,438 discusses a zoom lens having a zoom ratio of about 5. U.S. Pat. No. 6,185,048 discusses a zoom lens having a zoom ratio of about 3, in which a first lens unit is stationary and second, third and fourth lens units move during zooming.
In the four-unit zoom lens including the lens units having the above-described refractive power arrangement, the variable magnification is efficiently performed by moving the lens units in such a manner that a distance between the second lens unit and the third lens unit becomes maximum at a wide-angle end and becomes minimum at a telephoto end. Also, the reduction in the size of the entire zoom lens is achieved by configuring each lens unit with two or less lenses. In the zoom lens of this zoom type, if trying to achieve a high zoom ratio of 5 or more, a great variation occurs in chromatic aberrations such as axial chromatic aberration and chromatic aberration of magnification during zooming.
In the four-unit zoom lens described above, it is important to appropriately set the lens configuration of the respective lens units to achieve a high zoom ratio, reduce a variation in chromatic aberrations during zooming, and obtain superior optical performance over the entire zoom range. For example, it is important to appropriately set the number of lenses of each lens unit, refractive power of each lens unit, and an amount of movement of each lens unit during zooming. In particular, unless the lens configuration of the first, second and third lens units is appropriately set, a variation in chromatic aberration increases during zooming. Therefore, it is difficult to obtain superior optical performance.